Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as an energy source. Among these secondary batteries, lithium secondary batteries having high energy density and voltage are commercially available and widely used. The lithium secondary batteries generally use a lithium transition metal oxide as a cathode active material and a graphite-based material as an anode active material.
However, the anode formed of the graphite-based material has a maximum theoretical capacity of only 372 mAh/g (844 mAh/cc), thus suffering from a limited increase of capacity thereof. Consequently, such a graphite-based anode is incapable of carrying out a sufficient role as an energy source for next-generation mobile equipment undergoing rapid development and advancement. Further, lithium metals, studied for use as the anode material, have a very high energy density and thus may realize a high capacity, but raise problems associated with safety concerns due to growth of dendrites and a shortened cycle life as charge/discharge cycles are repeated. In addition, use of carbon nanotubes (CNTs) has been attempted as an anode active material, but various problems have been pointed out such as low productivity, expensiveness and low initial efficiency of less than 50%.
In this connection, a number of studies and suggestions have been recently proposed as to silicon, tin or alloys thereof, as they are known to be capable of performing reversible absorption (intercalation) and desorption (deintercalation) of large amounts of lithium ions through the reaction with lithium. For example, silicon (Si) has a maximum theoretical capacity of about 4020 mAh/g (9800 mAh/cc, a specific gravity of 2.23) which is substantially greater than the graphite-based materials, and thereby is promising as a high-capacity anode material.
However, upon performing charge/discharge processes, silicon, tin or alloys thereof react with lithium, thus undergoing significant changes of volume, i.e., ranging from 200 to 300%, and therefore repeated charge/discharge may result in separation of the anode active material from the current collector, or significant physicochemical changes at contact interfaces between the anode active materials, which are accompanied by increased resistance. Therefore, as charge/discharge cycles are repeated, the battery capacity sharply drops, thus suffering from a shortened cycle life thereof. For these problems, when a conventional binder for a graphite-based anode active material, i.e., polyvinylidene fluoride or styrene butadiene rubber, without any special treatment or processing, is directly applied to a silicon- or tin-based anode active material, it is impossible to achieve desired effects. In addition, when an excessive amount of a polymer as a binder is used to decrease volumetric changes occurring during charge/discharge cycles, separation of the active material from the current collector may be decreased slightly, but the electrical resistance of the anode is increased by an electrical insulating polymer used as the binder and the amount of the active material is relatively decreased, which consequently results in problems associated with a reduced battery capacity.
In order to cope with such problems, there is an urgent need for the development of a binder exhibiting superior adhesive strength and mechanical properties sufficient to withstand large volumetric changes of anode active materials occurring during a charge/discharge process in lithium secondary batteries using silicon- or tin-based anode active materials. In addition, conventional graphite-based lithium secondary batteries also require a strong need for the technique which is capable of improving the battery capacity by securing sufficient adhesion between the active material and the current collector and/or between the active materials, even with use of a small amount of the binder.
On the other hand, use of a polyvinyl alcohol has been attempted as a binder for an electrode of a lithium secondary battery (see Japanese Unexamined Patent Publication Nos. 1999-67216, 2003-109596 and 2004-134208). However, the above-mentioned polyvinyl alcohol binder exhibits superior adhesive strength, as compared to conventional binders, but suffers from a very low viscosity, non-uniform application of the binder on copper foil as a current collector and process problems associated with thermal treatment needed to improve adhesion between the electrode mix and the current collector. Further, the polyvinyl alcohol merely exhibits very insignificant effects to inhibit volumetric changes during charge/discharge cycling of the battery, due to a very low elongation percentage.
Other conventional prior arts focus on utilization of various polymers other than polyvinyl alcohols, as the binder.
For example, Korean Patent Application Publication No. 2002-011563 A1 discloses a lithium-sulfur battery exhibiting rapid electrochemical reaction, wherein a combination of polymers selected from a variety of polymers including polyvinyl alcohol and polyvinyl pyrrolidone is employed as the binder. Korean Patent Application Publication No. 2005-047242 A1 discloses an anode for a lithium secondary battery comprising an active material layer containing a polyolefin polymer and a water-soluble polymer, in which the water-soluble polymer is a thickening agent and is selected from the group consisting of carboxymethyl cellulose, polyvinyl alcohol, polyvinyl pyrrolidone and any combination thereof. Further, Korean Patent Application Publication No. 2005-085095 A1 discloses a binder composition comprising a vinylidene fluoride polymer having a functional group and/or a polar polymer having a carbonyl group, wherein the polar polymer includes at least one of an ethylene vinyl alcohol copolymer, a cellulose polymer, polyvinyl pyrrolidone and a vinylphenol polymer.
However, the aforementioned conventional arts merely exemplify various kinds of polymers that may be used as binders or active material additives, and do not suggest a mixture binder made of a certain combination in accordance with the present invention, as will be illustrated hereinafter. Further, none of the aforementioned prior arts teach or imply that selection of such a certain combination will bring about significant synergistic effects in adhesive strength and elongation percentage of the binder. In this connection, Experimental Examples that will be illustrated hereinafter presents analysis results of battery property and performance for the mixed binder of polyvinyl alcohol and polyvinyl pyrrolidone in accordance with the present invention, some binders of polymer combinations disclosed in the above-mentioned conventional arts, and a mixed binder of a low-molecular weight polyvinyl alcohol and polyvinyl pyrrolidone.
Meanwhile, when a conventional hydrogen storage electrode for an alkaline battery employs only the polyvinyl alcohol as the binder, adsorption of the polyvinyl alcohol to a hydrogen storage alloy powder results in separation of an electrode paste into the hydrogen storage alloy and water, which then leads to a decreased viscosity of the paste and significant changes in properties of the paste, thereby presenting problems associated with a deterioration of long-term storability of the paste and a decreased productivity of the hydrogen storage electrode. As an attempt to solve such problems, techniques involving addition of polyvinyl pyrrolidone are disclosed (see Japanese Unexamined Patent Publication No. 1998-040916 and U.S. Pat. No. 6,242,133). As an approach to solve the problem associated with an increased resistance of the electrode due to inhibition of ion migration resulting from coating of polyvinyl alcohol on the electrode, which is suffered by a paste-type cadmium electrode for an alkaline battery, there is also disclosed a technique of disrupting a polyvinyl alcohol-formed coating by further inclusion of polyvinyl pyrrolidone as a water-soluble sizing agent.
However, these techniques relate to binders that are incorporated into electrodes for alkaline batteries such as nickel-cadmium (Ni—Cd) batteries and nickel-hydrogen (Ni-MH) batteries. As compared to a lithium secondary battery, the alkaline battery exhibits a difference in active materials, electrolyte compositions, and the like, thereby leading to a difference in operation mechanisms during charge/discharge cycles. As a result, it is difficult to directly use the binder for electrodes of the alkaline battery in the lithium secondary battery, and practical applications of binders are also different therebetween.
Further, the adsorption problem of polyvinyl alcohol on the surface of electrodes of the alkaline battery occurs only when the polyvinyl alcohol having a low degree of polymerization is used. Such a fact can be clearly confirmed in working examples of the above Japanese Unexamined Patent Publication No. 1998-040916 wherein only the use of polyvinyl alcohol having a polymerization degree of 1500 is exemplified. Further, the inventors of the present invention have confirmed that the prolonged use of the polyvinyl alcohol having a polymerization degree of 1500 results in severe deterioration of the battery performance due to low electrolyte resistance, and particularly worsening of binder dissolution in the electrolyte upon continuous charge/discharge cycling at a high temperature. The secondary batteries easily reach a high temperature (for example, around 50° C.) during the continuous discharge process and the ensuing significant deterioration of the high-temperature performance may be an obstacle to impede the use of the secondary battery per se. Therefore, despite various suggestions of the conventional prior arts in connection with the alkaline batteries, the long-term use of the polyvinyl alcohol having a low polymerization degree in the lithium secondary battery has suffered from severe degradation of the battery performance, the fact of which can also be confirmed in the following examples.